1. Technical Field
The present invention relates generally to a method for determining conditions of a storage battery, and more particularly to an improved method for determining the amount of available reserve electric power remaining in an automotive storage battery to know a time when the battery should be recharged.
2. Background Art
An available capacity remaining in a battery is usually expressed in ampere-hours (AH), and determined by, for example, measuring current and voltage during discharge and looking up given data provided based on the measured current and voltage.
Japanese Patent First Publication No. 52-70234 teaches a simple battery capacity determining method utilizing a voltage level detected when a given battery current value is reached.
Generally, in a storage battery installed in vehicles, especially, employed as a power source of an electric vehicle, a discharged current and a battery voltage during vehicle traveling are frequently varied. The variation in the battery voltage, as shown in FIGS. 3(a) and 3(b), undergoes a delay following an abrupt variation in the discharged current. Thus, when a battery voltage is measured immediately after change in current, it will contain an error. This is because the specific gravity of electrolyte initially experiences a great reduction near electrodes, and then decreases wholly as a function of time.
Additionally, in an electric vehicle, the battery is recharged every time the vehicle is braked, leading to charge polarization of the battery. This will cause a battery voltage to be changed higher than a normal level. It, therefore, becomes difficult to determine a battery voltage correctly.
Further, continuous discharge of the battery also induces discharge polarization. This, as shown in FIG. 4, causes a battery consumption (AH) to be changed dependent upon an average current discharged so far if a battery voltage during discharge at given amperes (e.g., 200A) remains unchanged. Specifically, as the average discharged current increases, the battery consumption at a constant voltage, as shown by lines x, y, and z, tends to be decreased. Therefore, when the battery is discharged continuously with a great change in discharged current, it is difficult to determine a remaining battery capacity based on the battery voltage during discharge at given amperes (e.g., 200A).
Tie above phenomenon may be explained using a discharge characteristic of a constant current shown in FIG. 5. As can be seen in the graph, as an average discharge current is increased, a voltage drop, as shown by lines x, y, z, is developed earlier so that an available discharge duration is shortened, resulting in decreased available discharge capacity (discharge current x discharge duration). Thus, in this case, a remaining battery capacity cannot be used to determine battery conditions.